Telomeric integrity is required to maintain the replicative ability of cancer cells and is a target for the G-quadruplex–stabilizing drug 3,11-difluoro-6,8,13-trimethyl-8H-quino[4,3,2-kl]acridinium methosulfate (RHPS4). We report a senescent-like growth arrest in MCF-7 breast cancer cells, within 14 to 17 days, and a reduction in telomere length (from 5.2 kilobases (kb) to 4.7 and 4.3 kb after 17 days of treatment at 0.5 and 1 μM, respectively). These effects occurred at noncytotoxic drug concentrations (doses < 1 μM over a 14-day exposure) compatible with long-term drug dosing. The telomere length of cancer cells influences their sensitivity to growth inhibition by RHPS4: mutant (mt) human telomerase reverse transcriptase (hTERT)-expressing MCF-7 cells [short telomere restriction fragment (TRF) length, 1.9 kb; IC₅₀, 0.2 μM] were 10 times more sensitive to RHPS4 compared with wild-type (wt) hTERT-expressing, vector-transfected control cells (longer TRF-length 5.2 kb; IC₅₀ 2 μM) in the 5 day SRB assay. This relationship was corroborated in a panel of 36 human tumor xenografts grown in vitro showing a positive correlation between telomere length and growth inhibitory potency of RHPS4 (15-day clonogenic assay, r = 0.75). These observations are consistent with loss of the protective capping status of telomeres mediated by RHPS4 G-quadruplex-stabilization, thus leading to greater susceptibility of cells with shorter telomeres. In combination studies, paclitaxel (Taxol), doxorubicin (Adriamycin), and the experimental therapeutic agent 17-(allylamino)-17-demethoxygeldanamycin, which inhibits the 90-kDa heat shock protein, conferred enhanced sensitivity in RHPS4 treated MCF-7 cells, whereas the DNA-interactive temozolomide and cisplatin antagonized the action of RHPS4. Our results support the combined use of certain classes of cytotoxic anticancer agents with RHPS4 to enhance potential clinical benefit.